The present invention relates to fiber optic couplers, and more particularly, to couplers that exhibit low values of nonadiabatic-taper-induced excess loss.
This invention relates to fiber optic couplers regardless of their function or physical configuration. The various kinds of coupler function to which the invention applies include achromatic, wavelength division multiplexing, signal tapping, switching and the like. Examples of various configurations are: (a) fused biconic taper couplers that are made by heating and stretching a plurality of coextending optical fibers to fuse and taper them, (b) overclad couplers that are made by inserting a plurality of optical fibers into a tube and heating the tube to collapse it onto the fibers and thereafter stretching the tube midregion, and (c) coextending fiber couplers that are made by heating and stretching a plurality of optical fibers to taper them, and thereafter placing the stretched regions of the fibers adjacent one another to form a coupling region where, optionally, portions of the claddings have been removed by etching, polishing or the like. In the various kinds of couplers the coupling region is surrounded by a medium having a refractive index n.sub.3 that is lower than the refractive index of the coupler fiber cladding. The medium can consist of air, glass, plastic or the like.
As the requirements for the optical performance of fiber optic couplers become ever more stringent, the need to eliminate excess loss sources becomes more critical. One such loss source, which can be the dominant loss source in some couplers, is nonadiabatic-taper-induced excess loss.
In the tapered regions of fiber optic couplers the fundamental mode is continuously changing shape to accommodate the changing local index profile. If the rate of change of geometry is too great, the fundamental mode can be coupled to the higher order modes of the coupler index structure. This mechanism is referred to as nonadiabatic mode coupling. While the coupler is called a "single-mode" coupler, that actually refers to the fact that the input and output fibers only support the low-loss propagation of the fundamental LP.sub.01 mode. The coupler can typically support several bound, propagating modes. However, some of these modes may cut off at some point during the taper, coupling their optical power into radiation modes which are lost as potential coupler output, resulting in excess loss. Other higher-order modes which do not cut off will output their power into higher order modes of the output fibers. These modes suffer high loss and, again, the net impact is power lost to the coupler output and increased excess loss. Typically these nonadiabatic mode coupling effects are wavelength dependent, and excess loss varies as a function of wavelength.
The following symbols are used herein to characterize features of the prior art and/or the present invention. The term .DELTA..sub.1-2 is defined as (n.sub.1.sup.2 -n.sub.2.sup.2)/2n.sub.1.sup.2, where n.sub.1 and n.sub.2 are the refractive indices of the fiber core and cladding, respectively. The term .beta..sub.CR is used herein to mean the propagation constant of the fundamental mode in a coupler fiber in the coupling region of the coupler. The term .DELTA..sub.pedestal equals (n.sub.i.sup.2 -n.sub.2.sup.2)/n.sub.i.sup.2, where n.sub.i is the refractive index of that portion of the fiber just beyond the core (see refractive index dip 10 of FIG. 2 and refractive index pedestal 27 of FIG. 8).
In the refractive index profiles depicted in the figures, no attempt is made to represent indices and radii to scale and/or in exact relative magnitude.
Significant nonadiabatic mode coupling has been observed in a specific type of wavelength division multiplexing (WDM) coupler (referred to herein as a type A coupler) used to couple a signal (at wavelength .lambda..sub.S) and the pump power (at wavelength .lambda..sub.P) to the gain fiber of a fiber amplifier. One such coupler, which is disclosed in U.S. Pat. No. 5,179,603, functions as both a WDM and a mode field converter. A first coupler fiber has a core matched to a standard telecommunications fiber (.DELTA..sup.esi =0.36%, d.sub.c.sup.esi =8.3 .mu.m, mode field diameter=10.5 .mu.m at 1550 nm and 5.7 .mu.m at 1000 nm). The second coupler fiber has a large core-clad .DELTA..sub.1-2 (about 1%), a d.sub.c.sup.esi of 3.5 .mu.m, and a mode field diameter that is sufficiently small (6.4 .mu.m at 1550 nm and 3.7 .mu.m at 1000 nm) that it is substantially matched to an Erbium-doped gain fiber. The term .DELTA. .sup.esi is the equivalent step index delta of the fiber, and d.sub.c.sup.esi is the equivalent step index core diameter. The two coupler fibers would have possessed substantially different values of .beta..sub.CR except that the cladding of the second fiber is provided with an amount of chlorine that is greater than the amount of chlorine in the cladding of the first fiber, whereby the refractive index of the cladding of the second fiber is greater than that of the cladding of the first fiber. The refractive index disparity between coupler fiber claddings causes the .beta..sub.CR values thereof to become sufficiently matched that more than 95% of the light power at wavelength .lambda..sub.S couples between the first and second coupler fibers. Because of the manner in which the second fiber is made, its chlorine profile (idealized) is as shown in FIG. 1, and its overall refractive index profile (idealized) is as shown in FIG. 2. It is noted that the refractive index profile of these coupler fibers is determined by both the chlorine and germania doping, the large germania doping level at small radii (&lt;2 .mu.m) forming the inner refractive index peak shown in FIG. 2. The radius of transition between the two chlorine levels is r.sub.t, the core radius is r.sub.c and r.sub.o is the outside radius of the fiber.
The refractive index profile (idealized) of the standard telecommunication fiber is illustrated in FIG. 3.
This loss mechanism was observed to be even greater in WDM couplers (referred to herein as type B couplers) made with two identical small mode field diameter fibers of the type characterized by FIGS. 1 and 2. FIG. 4 shows excess loss versus wavelength for types A and B couplers (curves 12 and 14, respectively). In both cases, the variations in loss with respect to wavelength are indicative of possible nonadiabatic loss mechanisms.
It was discovered that type A couplers made with small mode field diameter fibers having a chlorine doping profile represented by curve 20 of FIG. 5 had 0.3 dB higher excess loss than those made with small mode field diameter fibers having a chlorine doping profile as represented by curve 21 of FIG. 5. Except for the cladding chlorine level, all other aspects of the small mode field diameter fibers were substantially identical. Thus, it became apparent that a larger dip in the chlorine profile caused larger coupler excess loss.